1. Field of the Invention
The present invention relates to a press forming process, more specifically, a press forming process involving stretching press to form a product with a curved portion from a workpiece by which a product with high dimensional accuracy can be easily produced.
2. Discussion of the Related Art
In recent years, there is a demand that many sorts of industrial machines including vehicles should have decreased weight and designed for safety. As a concrete measure to answer such demand, steel with high tensile strength is employed, which has excellent processing characteristics, weldability, and impact resistance in addition to the high tensile strength, whereby it has become possible to decrease weight of the product with maintaining the strength thereof.
However, when steel with high tensile strength is employed as a material for obtaining a product therefrom such as a car frame, and is manufactured by a press forming involving stretching and/or bending, a so-called xe2x80x9cspring backxe2x80x9d occurs at a curved portion to a larger extent in comparison with commonly employed steel. Thus, it is still difficult to improve the dimensional accuracy of a product by the press forming.
The reason why the xe2x80x9cspring backxe2x80x9d occurs is to be explained with referring to FIG. 6.
FIG. 6 shows a frame 1 extended to include a curved portion, and having an approximate U-shaped cross-section formed with lateral walls 3 at both sides of a top face 2. The curved portions 4 of the lateral walls 3 are obtained by a stretching press forming operation in order to prepare the frame 1 by the press forming. At this stage, the length of the frame I on an outer side of the bending is insufficient to retain a formed shape at the curved portion 4 due to a tensile force.
The short length at the curved portion 4 causes a spring back of the product. In FIG. 6, the normal shape of the frame 1 shown by alternate long and two short dashed lines is deformed at the side under the tension to the direction shown by arrows X, whereby an undesirable product shown by a solid line is obtained. Consequently, the dimensional accuracy of the product is decreased.
A new press forming process is proposed, for instance, as disclosed in Japanese Kokai Publication 7 (1995)-148527 wherein a product is neatly bent to prepare a corner by a part of a workpiece being bent once to have a corner with a depression, and then the corner with the depression is pressed again so as to have a sharp corner. In accordance with this method, the stress distribution on the bent portion can be well balanced in the plate thickness direction, so that the generation of the spring back phenomenon is suppressed.
Furthermore, Japanese Kokai Publication 11 (1999)-226651 proposes a method wherein the shape of a die is prepared so as to partially differ from the shape of a product to be attained therefrom by estimating deformation of the product, e.g. spring back or warp, followed by performing press forming to give a product, thereby maintaining accuracy of the product.
In the methods described in the above-mentioned Japanese Patent Kokai Publications 7 (1995)-148527 and 11 (1999)-226651, it is necessary to carry out press forming by the dies having shapes that are largely different from the products to be obtained. The deformation behavior of the products, however, does not always follow the estimation, so that it is necessary to repeatedly perform tests for determining the final shapes of dies, and many manufacturing steps are required also in the actual manufacture.
It is therefore an object of the present invention to provide a press forming process involving a stretching press to form a product with a curved portion from a workpiece, wherein a product with high dimensional accuracy can be obtained by an occurrence of a spring back thereof being restrained without using many different dies by performing a step of providing a pad portion to the workpiece prior to starting the press forming process, the formation of the being completed by spreading the pad portion mainly in a longitudinal direction of the workpiece, whereby the curved portion is prepared so as to have a spring back of the product minimized.
Another object of the present invention is to provide the above-mentioned press forming process involving a stretching press to form a product with a curved portion from a workpiece, by which the above-mentioned pad portion is easily created, by the pad portion being in a wave, a bead or a corrugation form in the above-mentioned press forming.
A further object of the present invention is to provide the above-mentioned press forming process involving a stretching press to form a product with a curved portion from a workpiece, by which the occurrence of a spring back is effectively restrained, by the workpiece being made of a steel with a high tensile strength.
A still further object of the present invention is to provide the above-mentioned press forming process involving a stretching press to form a product with a curved portion from a workpiece, by which press forming is smoothly carried out by preventing parts of the workpiece from catching or sticking to a die to be used therefor, by the pad portion having a shape which does not interfere with the die.
A still further object of the present invention is to provide the above-mentioned press forming process involving a stretching press to form a product with a curved portion from a workpiece, by which the deformation of the product by the spring back is surely restrained by a portion of the workpiece to be stretched being substantially the same as that of a face of a plate holding member of a die used of the stretching press.
A still further object of the present invention is to provide a product with a curved portion, which has high dimensional accuracy even at the curved portion without causing spring back, prepared by a press forming process from a workpiece, the press forming process comprising providing a pad portion to the workpiece prior to starting the press forming process, and completing formation of the curved portion by spreading of the pad portion mainly in the longitudinal direction of the workpiece, whereby the curved portion is prepared so as to have a spring back of the product minimized.
A still further object of the present invention is to provide a workpiece appropriately used for the above-mentioned press forming process to obtain a product with a portion which has high dimensional accuracy even at the curved portion without causing spring back, by the provision of a pad portion at the a portion of the workpiece.